Particulates (hereinafter also referred to as PM) such as soot in exhaust gas discharged from internal combustion engines including diesel engines and gasoline engines cause damage to environment and human bodies, which has been a problem these days. Since exhaust gas contains toxic gas components such as CO, HC, and NOx, there has also been a concern for adverse effects of the toxic gas components on the environment and human bodies.
In response, various filters having honeycomb structures (honeycomb filters) formed of porous ceramics such as cordierite and silicon carbide have been proposed as exhaust gas purifying apparatus. Such honeycomb filters are connected to internal combustion engines to capture PM in exhaust gas, or to convert the toxic gas components such as CO, HC, and NOx in the exhaust gas into nontoxic gas.
Recent exhaust gas regulations are increasingly strict for both diesel engines and gasoline engines, and there is a demand to provide an exhaust gas purification device that complies with such strict exhaust gas regulations. For example, exhaust gas purification devices including only a catalyst carrier carrying oxidation catalyst and a honeycomb filter that captures PM were sufficient enough to perform after-treatment of exhaust gas to a level that satisfies the exhaust gas regulations. However, due to an increased demand for NOx purification, exhaust gas purification devices are now required to include another catalyst carrier carrying SCR catalyst or a device such as an ammonia spraying mechanism to activate the SCR catalyst.
Based on such background, smaller catalyst carriers and smaller honeycomb filters are required on the premise that the above-described mechanism to activate catalyst is incorporated into exhaust gas purification devices. In particular, a honeycomb filter having a structure that can further suppress pressure loss is required.
Conventional honeycomb filters capable of reducing the pressure loss are disclosed in the following Patent Literature 1 to Patent Literature 4.
FIG. 8 (a) is a perspective view schematically showing a honeycomb filter disclosed in Patent Literature 1. FIG. 8(b) is a perspective view schematically showing a honeycomb fired body constituting the honeycomb filter.
As shown in FIG. 8(a) and FIG. 8(b), Patent Literature 1 discloses a honeycomb filter 90 formed by combining a plurality of honeycomb fired bodies 100 with one another with adhesive layers 105 therebetween and forming a peripheral coat layer 106 on the periphery of the aggregate of the combined honeycomb fired bodies, wherein the honeycomb fired bodies 100 each include exhaust gas introduction cells 102 each having an open end at the exhaust gas inlet side and a plugged end at the exhaust gas outlet side, and exhaust gas emission cells 101 each having an open end at the exhaust gas outlet side and a plugged end at the exhaust gas inlet side; the exhaust gas emission cells 101 each have a square cross section perpendicular to the longitudinal direction thereof; the exhaust gas introduction cells 102 each have an octagonal cross section perpendicular to the longitudinal direction thereof; and the exhaust gas emission cells 101 and the exhaust gas introduction cells 102 are alternately arranged with each other (a grid-like pattern).
Hereinafter, in the description of the present invention and the background art, a cell having an open end at the exhaust gas outlet side and a plugged end at the exhaust gas inlet side is simply described as an exhaust gas emission cell. A cell having an open end at the exhaust gas inlet side and a plugged end at the exhaust gas outlet side is simply described as an exhaust gas introduction cell.
The simple term “cell” means both the exhaust gas emission cell and the exhaust gas introduction cell.
Further, a cross section of the exhaust gas introduction cells and the exhaust gas emission cells in a plane perpendicular to the longitudinal direction thereof is simply described as a cross section of the exhaust gas introduction cells and the exhaust gas emission cells.
FIG. 9(a) is a perspective view schematically showing a honeycomb filter disclosed in Patent Literature 2. FIG. 9(b) is a view schematically showing an end face of the honeycomb filter.
Patent Literature 2 discloses a honeycomb filter 110 in which all the cells have the same square cross-sectional shape as shown in FIG. 9(a) and FIG. 9(b). In the honeycomb filter 110, exhaust gas emission cells 111 each having an open end at the exhaust gas outlet side and a plugged end at the exhaust gas inlet side are adjacently surrounded fully by exhaust gas introduction cells 112 and 114 across cell partition walls 113. The exhaust gas introduction cells 112 and 114 each have an open end at the exhaust gas inlet side and a plugged end at the exhaust gas outlet side. In the cross-sectional shape, each exhaust gas introduction cell 112 has one side that faces the exhaust gas emission cell 111 across the cell partition wall 113. In contrast, each exhaust gas introduction cell 114 faces the exhaust gas emission cells 111 in an angle-to-angle manner, and none of the sides forming the cross-sectional shape of the exhaust gas introduction cell 114 faces the exhaust gas emission cells 111.
FIG. 10 is a cross-sectional view schematically showing a cross section of a honeycomb filter according to Patent Literature 3.
As shown in FIG. 10, Patent Literature 3 discloses a honeycomb filter 120 including exhaust gas emission cells 121 having a hexagonal cross-sectional shape and exhaust gas introduction cells 122 having hexagonal cross-sectional shape. Each exhaust gas emission cell 121 has an open end at the exhaust gas outlet side and a plugged end at the exhaust gas inlet side. Each exhaust gas introduction cell 122 has an open end at the exhaust gas inlet side and a plugged end at the exhaust gas outlet side. Each exhaust gas emission cell 121 is adjacently surrounded fully by the exhaust gas introduction cells 122 across cell partition walls 123. It should be noted that the cross-sectional shape of each exhaust gas emission cell 121 is a regular hexagon, but the cross-sectional shape of each exhaust gas introduction cell 122 is a hexagon in which sides a and b with different lengths are alternately arranged with each other.
FIG. 11 is a cross-sectional view schematically showing a cross section of a honeycomb fired body constituting a honeycomb filter according to Patent Literature 4.
As shown in FIG. 11, Patent Literature 4 discloses a honeycomb filter 130 including exhaust gas emission cells 131 having an octagonal cross-sectional shape and two types of exhaust gas introduction cells with different cross-sectional shapes, i.e., first exhaust gas introduction cells 132 and second exhaust gas introduction cells 134. Each exhaust gas emission cell 131 has an open end at the exhaust gas outlet side and a plugged end at the exhaust gas inlet side, and the first exhaust gas introduction cells 132 and the second exhaust gas introduction cells 134 each have an open end at the exhaust gas inlet side and a plugged end at the exhaust gas outlet side. Each exhaust gas emission cell 131 is adjacently surrounded fully by the first exhaust gas introduction cells 132 and the second exhaust gas introduction cells 134 across cell partition walls 133. Specifically, these exhaust gas introduction cells include two types of cells: the first exhaust gas introduction cells 132 each having a square cross-sectional shape and the second exhaust gas introduction cells 134 each having an octagonal cross-sectional shape whose area is larger than that of the first exhaust gas introduction cell 132; and the cross-sectional area of each exhaust gas emission cell 131 is equal to or larger than the cross-sectional area of each second exhaust gas introduction cell 134.